Conditioned dogs are to be made hypertensive with an occluding cuff around one renal artery,with the other intact. As the dogs develop hypertension or not (sham), they will be anesthetized and their mesenteric vasculature perfused in vitro. New findings indicate that the veins undergo hypertrophy in the spontaneously hypertensive rat. Therefore, venous segmental and arterial segmental responses will be obtained; the mesenteric vasculature will then be perfused at in vivo distending pressures and the fixative, glutaraldehyde, substituted for blood. The muscles will be fixed at in vivo pressures and taken for electron microscopy and light microscopy. The ultrastructure of the muscles will be examined and compared between artery and vein within each animal and between normotensive and hypertensive animals. The experiments will be repeated, except prior to experimentation, when the animals are normotensive, they will be pretreated with either saline or clonidine and then allowed to develop either hypertension or normal blood pressure. In experiments performed under similar conditions, animals will be treated with radioactive precursors of cell membrane and nucleus constituents and the muscles removed, counted for radioactivity, and protein determined with conventional techniques. These data will provide information on the functional contractile, histological and ullrastructural, and biochemical properties of arteries and veins in hypertensive dogs and answer the question whether hypertrophic changes are primary defects resulting in altered vascular responses and the elevated pressures of dogs with two-kidney Goldblatt hypertension.